South Carolina and Nevada vote in US primaries

Voters in South Carolina have begun casting their ballots in the "First in the South" contest to pick the Republican nominee for the November 8 US presidential election.

Out west, Democrats gathered on Saturday across the state of Nevada for caucuses that marked the first test for Hillary Clinton and Bernie Sanders in a more racially diverse state than the opening contests in Iowa and New Hampshire, which the candidates split.

While Clinton's campaign once saw Nevada as an opportunity to start pulling away from Sanders, her team was nervously anticipating a close contest with the Vermont senator.

Trump ahead in polls

Opinion polls in South Carolina showed frontrunner Donald Trump solidified his spot at the top of the pack and rivals Ted Cruz and Marco Rubio fighting for a second-place finish.

Behind them, Republican candidates Jeb Bush, John Kasich and Ben Carson could be vying in South Carolina to keep their campaigns alive before the presidential race rapidly picks up steam in March when dozens of states hold nominating contests.

An NBC News-Wall Street Journal/Marist poll released on Friday showed Trump in the lead with support from 28 percent of likely Republican primary voters, followed by Cruz at 23 percent.

Rubio led Bush narrowly, 15 percent to 13 percent.

Voting in South Carolina will end at 00:00 GMT.

Clinton vs Sanders

For Democrats, the contest between Clinton and Sanders has grown closer than almost anyone expected.

Sanders, an avowed democratic socialist, has energised voters, particularly young people, with his impassioned calls for breaking up Wall Street banks and providing free tuition at public colleges and universities.

Clinton hoped to offset Sanders' youth support by winning big majorities among blacks and Hispanics. She eyed Nevada, where one-fourth of the population is Hispanic, as the first in a series of contests that would highlight that strength.

But Clinton's campaign has played down expectations in Nevada in recent days.

A victory for Sanders - or even a narrow loss to Clinton - would give his campaign a boost heading into the Democratic contests in 11 states on Super Tuesday.

Democrats were to gather at 200 caucus sites, including six at Las Vegas Strip casinos so housekeepers, blackjack dealers and others with weekend schedules could attend.

Democrats and Republicans will swap locations in the coming days. Republicans hold their caucuses in Nevada on Tuesday, while Democrats face off in South Carolina on February 27.